U.S. Pat. No. 4,859,361 to Reilly et al., the teachings of which are hereby incorporated by reference, describes a method of making electroluminescent phosphors which have a particle size ranging from 15 to 60 micrometers.
The performance of such phosphors is heavily dependent on the particle size of the phosphor. Generally, the brightness and halflife of the phosphor are greater if the particles are coarse, i.e., are larger than about 25 micrometers in diameter. Thus, fine phosphor particles (i.e., those having an average particle size of less than 25 micrometers in diameter) are undesirable because of inferior brightness and halflife characteristics.
It is common practice to screen the phosphor to ensure that only particle sizes of 25 micrometers or greater are provided. Typically about fifty to seventy percent of the phosphor production lot exceeds this particle size and can be used in electroluminescent display devices, while the remaining thirty to fifty percent of the lot is considered too fine to be useful. Thus, a significant portion of the phosphor yield is unsatisfactory for use in electroluminescent display devices.